Residents in Boston's North End will tell you that on a hot summer day you can still catch a whiff of molasses.

The entire neighbourhood was overrun with the stuff on one January day in 1919. A large tank of molasses burst open, sending waves of the sweet, sticky brown liquid crashing through the streets. Buildings collapsed and 21 people were killed.

But for nearly 100 years, the Great Molasses Flood has remained a great mystery. How, on a cold winter day, could a tank full of molasses move so quickly and cause so much damage?

Nicole Sharp has attempted to crack the mystery wide open. She's a science communicator and a fluid dynamics researcher in Cambridge, Mass. She recently presented her findings on the Molasses Flood at a conference in Portland, Ore.

Carol Off: Ms. Sharp, do we need to rethink that old saying "slow as molasses in January"?

Nicole Sharp: Well, if you have enough molasses, then yes. It turns out that saying can be wrong, if you have close to 9,000 cubic metres of molasses.

CO: So let's talk about this wave of molasses that left the tank that day in 1919. How quickly was the molasses travelling, as far as your calculations show?

NS: According to historical accounts, the molasses moved at a speed of 35 miles per hour, which is about 15.5 metres per second, which is really quite fast. Initially, I actually thought, "How in the world could it really have moved that fast?" and sort of surprised myself when I started doing the calculations. I realized that because the molasses is one and a half times denser than water and there was so much of it in this tank, in fact, fluid dynamics and the physics equations predict that it would move at speeds that are about 15 to 17 metres per second. It's totally in line with what the historical record says.

"Molasses, waist deep, covered the street and swirled and bubbled about the wreckage... only an upheaval, a thrashing about in the sticky mass, showed where any life was."- Report from The Boston Post

CO: So, as you understand it, if you can imagine what was happening when that tank burst, how did it start to flow into the city?

NS: For the first 30 to 60 seconds or so, according to our calculations, that wave would have moved a lot like a tsunami, if you could imagine that. In that moment, what mattered about the molasses was its weight and heavy density. So, for that initial minute, you just have this massive wave of heavy fluid that's crashing through everything. After that first minute or so, that's when the fact that it's molasses, and molasses is incredibly viscous, so it's really thick and it likes to try to resist flow, that's when that starts coming into play. Then you go from being a sort of tsunami to being like a seeping. It's a seamless transition between those, but the longer you go after that first minute the more the viscosity of the molasses matters and the more it's going to kind of seep through the neighbourhood and creep.

NS: It seems like that would be the point where it's not as dangerous any more. It's not crashing through buildings after that first minute. It's just kind of oozing instead. But it turns out that, because it was cold and because the molasses was cooling, that, if anything, that made the molasses more dangerous. Now people who have been knocked down by that initial wave who may have been pinned in wreckage are trapped in places where they have to try to keep this molasses away from their mouth and nose so they can breathe while people are trying to come and get them. That extra cold makes the molasses easily four or more times as viscous as before and that makes it much harder to fight.

An elevated train structure was damaged by shrapnel in the disaster. (Wikipedia)

CO: A
Boston Post report at the time described it as "molasses, waist deep, covered the street and swirled and bubbled about the wreckage . . . only an upheaval, a thrashing about in the sticky mass, showed where any life was." People, horses, everything just struggled to survive and died. It's extraordinary.

NS: Absolutely. It's a frightening thought really. I think the group of men who were having lunch is one of the more striking stories. That was a group of men who were in a firehouse a short ways away from the molasses tank. The initial wave smashed against the firehouse and pushed it almost entirely off its foundation. Then the upper floors of the firehouse crashed down on them. I believe they were on the first floor and they were trapped there for hours waiting for people to be able to rescue them because they were all pinned in place.

CO: And how many of those men survived that?

NS: All but one of them survived. One of them survived for a couple of hours and then he couldn't continue fighting the molasses anymore and he ended up drowning.

This interview has been edited for length and clarity. For more on this story, listen to our full interview with Nicole Sharp.

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